Super type of List<@Nullable Something> and List<@NonNull Something>

[William Shackleford]

Let's say one has a method that only reads from a list and when  it gets something from

the list it checks that it is not null before using it.

It is therefore safe to call this method with a List<@Nullable Something> since it is guaranteed not to throw an exception even if there are nulls on the list. It is also safe to call

with List<@NonNull Something> since it will never put a null value onto the list.

Do I have to create two methods with identical content to handle both cases or is

there a way to effectively declare the method to take a parameter which is the super type of both?

[Michael Ernst]

You can write a single method.  Two mechanisms that you could use are Java wildcards and type qualifier polymorphism.  Both are described in chapter 30, "Generics and polymorphism", in the manual (https://checkerframework.org/manual/#polymorphism).

Below is a code example showing both approaches.

                    -Mike

   
import org.checkerframework.checker.nullness.qual.PolyNull;
import org.checkerframework.checker.nullness.qual.NonNull;
import org.checkerframework.checker.nullness.qual.Nullable;
import java.util.List;

public class Foo {

  List<@Nullable String> nbleStrings = List.of("a", null, "c");
  List<@NonNull String>  nnStrings = List.of("a", "b", "c");

  void client() {
    library(nbleStrings);
    library(nnStrings);
    library2(nbleStrings);
    library2(nnStrings);
  }

  void library(List<? extends @Nullable String> strings) {
    for (String s : strings) {
      if (s != null) {
        s.hashCode();
      }
    }
  }

  void library2(List<@PolyNull String> strings) {
    for (String s : strings) {
      if (s != null) {
        s.hashCode();
      }
    }
  }
}

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